What Are the Causes and Treatment of Bilateral Nephrolithiasis?

Bilateral nephrolithiasis simply means stones in both kidneys, and in most people it grows from the same urine chemistry that causes stones on one side. It is also much less common than a stone on one side, found in under 1 in 100 adults in one large health-check population. Treatment has two jobs: clearing the stones now, and finding the cause so the next round doesn’t follow.

Why Stones Form in Both Kidneys

The most common stone in people with stones on both sides is calcium oxalate, which makes up nearly half of the main stone types.¹ Oxalate is a natural compound that comes from food and from the body’s own chemistry, and high levels of it in urine have long been known as a major driver of stones.² Anything that raises oxalate or calcium in the urine, or lowers the substances that protect against crystals, acts on both kidneys at once.

Both kidneys filter the same blood, so they make the same urine chemistry, and a stone on one side is often joined later by one on the other. Low fluid intake is the shared background: it concentrates urine and lets crystals form in both kidneys. Hot climates, physical jobs and heavy sweating raise that risk, which is why drinking more is the first advice whatever the cause turns out to be.

Some people are more likely than others to end up with two-sided disease. Men are about one and a half times as likely as women to have stones on both sides rather than one.³ High blood uric acid raises the chance by a similar amount.⁴ People with stones in both kidneys also more often have high blood pressure and high blood sugar.⁵

They also tend to start earlier. People with stones on both sides typically get their first stone around age 37, about nine years earlier than one-sided formers.⁶

The stones on each side often form on different timelines, so one kidney may hold a stone found by accident while the other is the one causing pain. Because the cause usually lies in blood and urine chemistry, removing the stones from one side doesn’t change the risk of new stones on either side.

Underlying Conditions That Point to Stones on Both Sides

Inherited conditions are one route. In one genetic study, about 7 in 100 adults having stone procedures carried an inherited cause such as cystinuria, a kidney acid problem, or phosphate loss.⁷ These cases tended to start young, come back often, and affect both kidneys.⁸ Cystine stones, the hallmark of cystinuria, were more common in two-sided stone formers, though still under 2 in 100.⁹ Someone with early, repeated stones on both sides has a better reason than most to ask about genetic testing.

Hormone and acid-balance problems come next. An overactive parathyroid gland pushes extra calcium into the urine, which can deposit calcium in the kidneys.¹⁰ That damage can upset how the kidneys handle acid, which itself favors stones.¹¹ Blood calcium and parathyroid hormone are worth checking when stones affect both sides.

The gut matters too. After weight-loss surgery the gut absorbs more oxalate, and urine has less citrate, a natural stone blocker.¹² In people after such surgery, a low-oxalate diet halved urine oxalate, while cutting dietary calcium sharply doubled it.¹³ Some gut bacteria also break down oxalate before it reaches the urine, and this involves several species, not one.¹⁴

Kidney structure and medicines can play a part as well. Medullary sponge kidney, a structural difference inside the kidneys, is linked to repeated stones, though it is sometimes mislabelled.¹⁵ Some medicines can form stones themselves, including ciprofloxacin, sulfa drugs, triamterene, indinavir, and ephedrine with guaifenesin.¹⁶ Stopping the drug is usually needed, but only with your doctor’s agreement.

To look for a cause, a doctor will ask how old you were at your first stone and how many you’ve had since, because an early start and frequent stones raise suspicion of an inherited or metabolic cause. Relatives with stones or kidney disease can point toward a genetic condition. Past bowel or weight-loss surgery, and your current medicines and supplements, are reviewed because both can change what ends up in your urine.

Keeping a passed stone helps here. Analysis shows whether it is calcium, uric acid or cystine, and each type points to a different cause and different prevention. Straining your urine through a sieve or coffee filter catches fragments you would otherwise flush away.

Is It More Serious Than a Stone in One Kidney

Stones in both kidneys do come back more. People with stones on both sides had on average about 11 stone episodes, compared with 3 for one-sided formers.¹⁷

Kidney function is also at stake. Chronic kidney disease was found in about 3 in 10 people with two-sided stones, versus about 2 in 10 with one-sided stones and 1 in 10 without stones.¹⁸ Repeated stones, past stone surgery and urine infections were also linked to kidney disease.¹⁹ People under 50 who had more than two stone operations showed a clear drop in kidney filtering.²⁰ Each procedure and each blockage can leave small amounts of scarring, so preventing future stones cuts the number of procedures, and that is the main reason doctors push follow-up tests. Your lab report’s eGFR, a kidney filtering score that estimates how well your kidneys clean the blood, is worth tracking each year.

Blocked stones on both sides can become an emergency. If stones block both kidneys, or a single working kidney, the kidneys can suddenly fail, which is a urological emergency.²¹ Prompt treatment works well, though: when treated quickly with a scope, kidney function returned to normal within a week in about 94 in 100 such patients.²²

Several signs mean going to emergency care. Passing very little or no urine is a red flag, because with stones on both sides it can mean both kidneys are blocked. Severe pain together with fever or shaking chills suggests an infected, blocked kidney, which needs urgent treatment. Vomiting that stops you keeping fluids down is a reason to seek care, since dehydration can worsen things. Pain on both sides at once, with any of these signs, should not be waited out.

How Stones in Both Kidneys Are Removed

Ureteroscopy passes a thin scope up through the bladder to break stones with a laser, with no cuts in the skin. Treating both kidneys in one session this way is considered safe and effective for selected patients, especially with total stone size under 25 mm.²³ For stones up to 1.5 cm, about 87 in 100 were stone-free after one session and 97 in 100 after a second.²⁴ Complications do happen: across 1,250 real-world patients, about 1 in 10 had a short fever and 2 in 100 developed a serious blood infection.²⁵

One general anesthetic usually covers both kidneys, so you sleep through the whole session. Surgeons often leave small tubes called stents in the ureters to keep urine draining while swelling settles, and these come out or are exchanged at a later visit. Blood in the urine and a frequent urge to pee are common for several days, often from the stents. A fever afterward should prompt a call to the surgical team, because it can signal an infection that needs prompt treatment.

Large stones call for PCNL (percutaneous nephrolithotomy), which removes them through a small tunnel made in the back into the kidney. Doing both sides in one operation cleared stones in about 8 in 10 kidneys, with a typical four-day hospital stay.²⁶ In another comparison, staged surgery cleared stones more often, about 65 versus 44 in 100, and needed fewer transfusions.²⁷ Whether to do both sides together depends on stone size and the patient’s health, and it’s reasonable to ask your surgeon why they recommend one approach.

Surgeons sometimes stage the sides for practical reasons. Doing one side first means that if something goes badly, the other kidney is still working normally. Very large stones on both sides can make a single long operation harder on the patient, so splitting it can be safer. An infection or reduced kidney function may favor treating the more affected side first and the other later.

Shock wave lithotripsy (ESWL) breaks stones from outside the body without any scope. Doing both sides together carries a risk of blocking both kidneys with fragments.²⁸ About 16 in 100 needed another procedure, mostly with stones 20 mm or larger.²⁹ Long-term kidney function, however, was similar whether both sides were treated together or one at a time.³⁰

Tests to Find Your Cause

Guidelines recommend a full metabolic workup for people at high risk of repeat stones.³¹ Stones in both kidneys put you in that group. Diet and medicine chosen from these results, with close follow-up, are what reduce return.³²

The centerpiece is a 24-hour urine test. You begin after emptying your bladder in the morning, discard that first sample, then collect every drop in the container for the next 24 hours, including the first sample the following morning. Eat and drink as you normally do, so the results reflect your real habits rather than a best-behavior day. The urine collection measures total volume plus calcium, oxalate, citrate and uric acid, which show which stone-promoting factors are at work. It is often done twice, and repeated after treatment or diet changes to check whether they are working.

Blood tests usually come with it. Calcium and parathyroid hormone look for an overactive gland, uric acid is checked, and so are kidney function and blood acid balance.

Genetic testing has a narrower role. Outside rare inherited conditions such as cystinuria or primary hyperoxaluria, no test can predict how your stones will behave.³³ It is most useful when stones start young, recur often, or run in the family.

Preventing New Stones

Preventing return works best by combining diet and medicine tailored to the person.³⁴ The foundation is fluid: aim for about 2.5 litres of urine a day, which usually means drinking around 3 litres. Pale urine, not dark yellow, is a good sign. Drink more on hot days and during exercise, and spread fluids through the day, including the evening. Follow-up urine tests confirm whether your volume is enough.

Don’t cut calcium in food, because dietary calcium binds oxalate in the gut.³⁵ Pair high-oxalate foods like spinach and nuts with dairy at the same meal. Cutting dairy to avoid calcium stones backfires, because less calcium in the gut means more oxalate gets through. A high-salt diet raises the calcium in urine, so reducing salt helps even when you eat plenty of calcium. Large amounts of meat raise uric acid and lower citrate, which favors stones. Sugary sodas add risk, while lemon in water supplies citrate.

When tests show a specific problem, medicine fills the gap. Thiazide water pills lower urine calcium, potassium citrate raises citrate, and allopurinol lowers uric acid. Cystinuria needs very high fluid intake and urine-alkalizing medicine.

Stones in Both Kidneys in Children and Pregnancy

Infants more often have stones on both sides, and multiple stones, than older children.³⁶ About two-thirds of such infants had low urine citrate and high oxalate at follow-up, so repeated testing is needed.³⁷

In pregnancy, ultrasound is the first scan, with MRI used to tell the normal widening of the kidneys from a stone blockage.³⁸ Kidney stone pain in pregnancy carries risks for mother and baby and needs special care.³⁹ Tell the maternity team promptly about flank pain or fever rather than waiting for it to pass. Because normal pregnancy widens the kidney drainage system, a scan can look like a blockage even when no stone is present, so doctors interpret results carefully. When a stone is blocking drainage, treatment is often a temporary stent or drain that stays in until after the birth.